1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is capable of displaying images on a large screen by tiling a plurality of liquid crystal display panels or switching array substrates.
The invention also relates to a liquid crystal display device having a large display screen capable of displaying images with high quality.
2. Description of the Related Art
Demands for a large-screen display device are increasing these years. For example, demands for a large-screen television set for home use are high. Those display allows to promote a quality and gives a realism of an experience of watching a TV. However, when a conventional CRT is made to have a larger screen, its volume and weight increase. Thus, it is highly focused to supply a large-screen flat panel display, which can be made thin and lightweight.
A liquid crystal display device is one of promising device among the flat panel display devices. However, it is difficult to achieve a large 40-inch class display by a single liquid crystal panel in view of production facilities and productivity.
Under such circumstances, there have been made several proposals including a method of combining a single large screen by connecting a plurality of liquid crystal panels.
For example, Japanese Patent Laid-Open Application No. Hei 8-76074 proposes a method of directly connecting liquid crystal panels in parallel to each other. This method has an advantage that a large screen can be made with taking full advantage of a thin feature of the liquid crystal display devices as it is.
Where liquid crystal panels are directly connected in parallel to each other, display quality of a formed display screen depends highly on a width of the connecting region between the liquid crystal panels. When the connecting region has a large width and the display region of the connected panels is divided, the connecting region is recognized as a disturbing line to the observer. This is because the connecting region is a non-display region.
To fulfill such conditions, Japanese Patent Laid-Open Application No. Hei 8-76074 proposes a method of forming a width, which is equal to or larger than a width required for connecting liquid crystal panels, as a black matrix (BM) region between picture elements of the individual liquid crystal panel.
In the LCD, to prevent the connecting region from being recognized visually, a width of BM on the display region other than the connecting region is required to be equal to a width of BM on the connecting region. When a width of BM in a direction parallel to a data line of a single liquid crystal display panel is taken into account, BM of the connecting region has a width equal to a summary of a margin for connection and a seal width. Therefore, a connecting region of about 300 .mu.m or more is required under the present conditions.
Meanwhile, every BM except the connecting region is required to have a width to cover the data line and a area between the data line and the pixel electrode. Therefore, its width is preferably about 30 .mu.m.
In other words, a liquid crystal display device having a large screen formed by directly connecting liquid crystal display panels in parallel requires making the width of every black matrix except the connecting region with a width wider than a preferable width.
A conventional liquid crystal display device having liquid crystal display panels directly connected in parallel to each other will be described with reference to FIG. 19.
FIG. 19 is an enlarged schematic diagram of a pixel part in a region other than a connecting region of a display region. Reference numerals 91R, 91G, 91B denote pixels R, G, B, and these three pixel electrodes form one picture element. To the respective pixel electrodes are connected TFTs 92a, 92b, 92c which are non-linear switching elements, and the three TFTs 92a, 92b, 92c belonging to one picture element are connected to the same gate line 94. These three TFTs 92a, 92b, 92c are also connected to different data lines 93r, 93g, 93b.
The respective pixel electrodes, the TFTs connected thereto and the data lines have the same positional relationship of three R, G and B. However, between the neighboring picture elements, a non-illustrated black matrix (BM) with width L.sub.BM is formed to cover the non-display region of the connecting region. Since there is a data line between the neighboring pixels in a single picture element, width LS of the black matrix between the adjacent pixels needs to be about 30 .mu.m in order to cover the data line and the region between the data line and the pixel.
When it is assumed that a pitch in a direction of the gate line of the picture element is about 600 .mu.m, the black matrix between the picture elements has width of L.sub.BM of 330 .mu.m, and BM between the pixels has width Ls of 30 .mu.m, an optical transmission region of the pixel in a direction of the gate line has length Lp of 70 .mu.m, a ratio of portions where light passes through becomes small, and light transmittance of the liquid crystal display device becomes small.
When the liquid crystal display device has a large screen by directly connecting liquid crystal panels in parallel to each other and a yield of the respective liquid crystal panels or switching array substrates to be connected is different, a total yield of the liquid crystal display device as the whole is limited to a yield of a substrate having the lowest yield.
Furthermore, the respective liquid crystal panels have individual differences in display characteristics and, therefore, there is a problem that flexibility of combining switching array substrates having a small difference in characteristics is lowered when a direction of the switching array substrates to be connected is specified.
It is conspicuous that a quantity of display information on the display device tends to increase. In addition, in order to deal with the increase of a quantity of display information, a display device having a larger screen and higher resolution is being demanded.
However, the liquid crystal display device has a disadvantage that writing time for driving the liquid crystal becomes short when the liquid crystal display device has higher resolution. Additionally, when the display screen is made larger, the pixel and the data line have an increased capacitance, and a wave profile of the data signal being dull. Thus, the data signal cannot be applied completely to respective pixels, and a contrast of displaying image is degraded.